people who are fans of pro-wrestling and they and other friends simulate the moves they see on either a trampoline or mattress.
the "tard" part is used because it is seen as a stupid act and it is illegal.

Backyard wrestlers who do not understand the concept of psychology in professional wrestling matches and instead resort to hitting each other with random objects such as light tubes, street signs, barbed wire, etc. and jumping off of high places onto each other.

"Did you see that guy set his opponent on fire in the movie The Backyard?"
"Yeah, he's a yardtard."

The Urban Dictionary T-Shirt

A lowbrow practitioner of backyard wrestling who will often perform high risk maneuvers and use crude weaponry. 1998-2001 was considered the heyday of yardtards. Subsequently, there has been a growing movement of mixed martial artists style yardtards who often emulate matches from The Ultimate Fighting Championship as the show started to become broadcasted on cable tv following the reality show The Ultimate Fighter. Many backyarders who follow this trend like to brawl in their backyards or garages after buying heavy bag gloves from sporting goods stores. These backyarders are frequently called "TUF noobs" for they do not formally train martial arts at academies and have only become followers of mixed martial arts following the reality tv show. The highest extent of an mma yardtard's training usually is high school wrestling and practicing techniques sought online.

"I saw a Tapout and Bad Boy sticker on Trey's lifted Ford f-150. Does he train jiu-jitsu or mma?"

"Naw, he's just one of those newbreed yardtards who wrestled in high school and does 'garage-jitsu'."